1. Field of the Invention
The present invention relates to an improvement of a sol of modified colloidal particles having a particle size of from 4.5 to 60 nm, formed by coating the surface of metal oxide colloidal particles with colloidal particles of tungstic oxide-stannic oxide composite having a particle size of from 2 to 7 nm. The sol of the present invention is useful as a component of a hard coat agent to be applied on the surface of plastic lenses and for various other applications.
2. Description of the BAckground
Heretofore, various sols of metal oxides have been known. A sol of a metal oxide having a high refractive index is used as a component of a hard coat agent to be applied on the surface of plastic lenses for the purpose of improving the surface of such plastic lenses which have been widely used in recent years.
For example, Japanese Examined Patent Publication No. 37142/1988 discloses a hard coat agent containing from 1 to 300 nm particles of an oxide of metal such as Al, Ti, Zr, Sn or Sb.
Japanese Examined Patent Publication No. 40119/1975 proposes a silicic acid-stannic acid composite sol wherein the molar ratio of Si:Sn is from 2:1 to 1,000:1.
A sol of tungstic oxide alone which is stable for more than one hour, has not yet been known, but Japanese Unexamined Patent Publication No. 52686/1979 proposes a sol of WO.sub.3 :SiO.sub.2 :M.sub.2 O wherein M is an alkali metal atom or an ammonium group in a molar ratio of 4-15:2-5:1 which is obtained by an addition of a silicate.
Japanese Unexamined Patent Publication No. 151038/1991 discloses a sol of tungstic oxide-stannic oxide composite. This sol contains tungstic oxide and stannic oxide in a WO.sub.3/ SnO.sub.2 weight ratio of from 2 to 100 and contains M.sub.2 O wherein M is an alkali metal atom, an ammonium group or an amine molecule in a molar ratio of from 0.02 to 0.7 to the sum of WO.sub.3 and SnO.sub.2.
Japanese Unexamined Patent Publication No. 217230/1991 proposes a stable sol which comprises colloidal particles of modified metal oxide having a particle size of from 4.5 to 60 nm and formed by coating colloidal particles of tungstic oxide-stannic oxide composite having a particle size of from 2 to 7 nm and a WO.sub.3 /SnO.sub.2 weight ratio of from 0.5 to 100 on the surface of colloidal particles of trivalent, tetravalent or pentavalent metal oxide having a particle size of from 4 to 50 nm as nuclei, and which has a total content of such metal oxides of from 2 to 50 wt %.
However, when these conventional metal oxide sols, particularly cationic metal oxide sols, are used as components of hard coat agents, not only the stability of the hard coat agents thereby obtained, will be inadequate, but also the transparency, the adhesion properties or the weather resistance of the cured coating films of such hard coat agents, will be inadequate.
When a Sb.sub.2 O.sub.5 sol is used as a component of a hard coat agent, if the refractive index of the plastic base material of the lens is 1.6 or more, adequate improvement in the refractive index of the cured coating film can no longer be expected with this Sb.sub.2 O.sub.5 sol, since the refractive index of Sb.sub.2 O.sub.5 is at a level of from 1.65 to 1.70.
The sol of tungstic oxide disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 52686/1979, is obtained by adding a silicate to an aqueous solution of tungstic acid obtained by subjecting an aqueous solution of a tungstate to cation-removing treatment, but it is stable only under a strongly acidic condition. Further, even if this is used as a component of a hard coat agent, no substantial effects for improving the refractive index of the coating film will be obtained.
The silicic acid-stannic acid composite sol disclosed in the above-mentioned Japanese Examined Patent Publication No. 40119/1975, is obtained by subjecting an aqueous solution of a mixture comprising an alkali metal silicate and an alkali metal stannate to cation-removing treatment. However, like in the above case, even if this sol is used as a component of a hard coat agent, no substantial effects for improving the refractive index of the coating film will be obtained.
The colloidal particles of the tungstic oxide-stannic oxide composite sol disclosed in the above Japanese Unexamined Patent Publication No. 151038/1991 have a refractive index of from 1.8 to 1.9, and a hard coat film containing them is excellent in the scratch resistance, the transparency and the adhesion property, but its water resistance is inadequate.
The colloidal particles of the modified metal oxide sol disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 217230/1991 have a refractive index of at least 1.7, and when this sol is used as a component of a hard coat agent for a plastic lens, it is possible to substantially satisfy the properties required for a hard coat film such as the scratch resistance, transparency, the adhesion property, the water resistance and the weather resistance. However, when the nuclei of such modified metal oxide colloidal particles are a stannic oxide sol, a hard coat film obtained by using such a sol, is susceptible to yellowing when exposed to ultraviolet rays.